Biaxially oriented polypropylene films are made by co-extruding polypropylene granules into sheets, followed by stretching both transversally and longitudinally. The biaxially oriented polypropylene films have better mechanical property, high transparency and glossiness, and are currently the most widely used printed film.
In order to make the biaxially oriented polypropylene films have more excellent mechanical property, barrier property or heat sealability property and other properties, the biaxially oriented polypropylene films are typically subjected to be coated to produce coated films. When traditional biaxially oriented polypropylene films are coated as substrate, in order to deal with the problem that surface coating polyvinylidene chloride and other coatings are difficult to be firmly attached to film substrate, typically, the films are first subjected to corona treatment so that the surface tension reaches 38-42 dynes, solvent type polyurethane and other adhesives are then pre-coated, after drying, polyvinylidene chloride and other surface coatings are coated. The technological processes are as follows: corona treatment of substrate surface—pre-coating of primer—drying—surface coating—drying.
However, primer coating has the following defects: (1) the vaporization of toluene, ethyl acetate, and other solvents used in solvent type primer during the production process causes pollution to the surrounding environment, and also endanger the health of operators; (2) residues of toluene, ethyl acetate, and other solvents on coated film contaminate foods, drugs, etc. packaged therein, which endanger the health of consumers; (3) toluene, ethyl acetate, and other solvents are all inflammable and explosive materials, great safety risks exist in the storage of solvents; (4) drying equipment is required to match the pre-coated primer, which increase the consumption of fuel gas, electricity power and other energy, and therefore the production costs are higher. Although the environmental issue has been improved to some extent with the development of water-based adhesives in recent year, the problem always exists, that is the adhesive ability of water-based adhesives decreases with the increase of environmental humidity, and the costs are still very high.